Please say a bit more about that - are you saying that for the case of tubing embedded in concrete that you think convection is the dominant mode, or that convection is dominant only in the case of tubing in air?

I meant to say that you were comparing apples to oranges. In the case of tubes embedded in concrete, thermal conductivity is the dominant heat transfer mode. Although forced convection is the mode of heat transfer from the water to the pipe, the heat transfer from the pipe to the concrete to the floor is all conduction.

In the tubing in air system, the dominate mode of heat transfer from the pipe to the air is convection. In turn natural convection (or more precisely, buoyancy) causes the air in the space to circulate, and then finally convection is the heat transfer mode between the air and the floor.

Comparing the thermal conductivty of air to that of concrete is not relevant here since the air transfers heat in this situation by convection, not conduction.